Device for forming vertically aligned carbon nanotube arrays

ABSTRACT

A method and device for producing an aligned carbon nanotube array. The arrays of aligned carbon nanotubes (CNTs) may be formed by drying liquid dispersions of CNTs on a nanoporous substrate under an applied electrostatic field. The array may be used in a number of applications including electronics, optics, and filtration, including desalination.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional application claiming the benefit of U.S. patentapplication Ser. No. 12/894,043 filed Sep. 29, 2010, which isincorporated herein by reference in its entirety as if fully set forthherein.

U.S. GOVERNMENT RIGHTS

The invention described herein was made under United States GovernmentContract Number 71328. The government may have certain rights in thisinvention.

FIELD

This invention relates to a method and device used to produce verticallyaligned carbon nanotube arrays. In particular, the present inventionrelates to a method, array, and device used to produce verticallyaligned carbon nanotube arrays from liquid dispersions.

BACKGROUND

Carbon nanotubes and nanowires are noted for their useful and novelproperties in electrical, chemical, optical, and filtrationapplications, among others. Often, such nanotubes are grown in bulk andhave non-uniform diameters. Accordingly, it would be desirable toprovide methods for positioning and orienting nanotubes into arrays on avariety of substrates or within a variety of integrated devices.Existing efforts to align nanotubes for such applications are oftendifficult and labor intensive. Therefore, in situ efforts to alignnanotubes during the growth process have been attempted.

In one such example, a porous substrate or filter is used to align thenanotubes as they are grown. In this example, the diameter of alignedcarbon nanotubes and the density of the array are limited. Typically,very high temperatures are necessary to grow the carbon nanotubes.Accordingly, any substrates or other apparatus used to make a carbonnanotube array must be specially constructed to survive such hightemperatures. Other issues arise with the difficulties of purifyingand/or isolating nanotubes grown in an array based on certaincharacteristics, such as length or diameter.

Thus, a need exists for a method and apparatus for forming aligned,vertically or otherwise, carbon nanotubes at room temperatures on avariety of substrates where the carbon nanotubes can be purified andselected based on desired characteristics, such as length and/ordiameter.

SUMMARY

The present disclosure provides methods for producing an aligned carbonnanotube array and an aligned carbon nanotube array useful in a numberof applications. In one embodiment, a method for forming an alignedcarbon nanotube array includes applying an electrostatic voltage acrossa solution containing carbon nanotubes. The carbon nanotubes are thenattached to a porous substrate.

In another embodiment, a method for forming an aligned carbon nanotubearray includes passing a solvent containing at least one suspendedcarbon nanotube through a porous substrate. An electrostatic voltage isapplied across the porous substrate and the solvent is evaporated as theelectrostatic voltage is applied. A binder is then applied to the poroussubstrate to bind the at least one carbon nanotube to the poroussubstrate.

One embodiment of the method for forming a vertically aligned carbonnanotube array includes depositing carbon nanotubes suspended in asolvent onto the surface of a nanoporous substrate. An electrostaticvoltage is applied perpendicular to the surface of the nanoporoussubstrate to align the carbon nanotubes perpendicular to the surface.The solvent is evaporated as the electrostatic voltage is applied. Thena binding solution is applied to the nanoporous substrate. The bindingsolution binds the perpendicularly aligned carbon nanotubes to thenanoporous substrate.

In yet another embodiment, the method for forming vertically alignedcarbon nanotube arrays uses carbon nanotubes of a particular diameter.The method includes suspending grown carbon nanotubes in a liquid andthen oxidizing the carbon nanotubes in the suspension usingultrasonication. The suspension is spun in a centrifuge to isolate thecarbon nanotubes. A fraction of the suspension containing the carbonnanotubes is collected and at least one surfactant is added to thefraction. The surfactant bonds to the carbon nanotubes having theparticular diameter, and the fraction is then spun to isolate the carbonnanotubes having the particular diameter. Another fraction of thesuspension containing the at least one surfactant and the carbonnanotubes having the particular diameter is collected. The otherfraction is then deposited onto a nanoporous substrate. Next, anelectrostatic voltage is applied perpendicular to the surface of thenanoporous substrate. The electrostatic voltage aligns the carbonnanotubes perpendicular to the surface of the nanoporous substrate. Theliquid is evaporated from the nanoporous substrate as the electrostaticvoltage is applied. Finally, a hydrocarbon-based binding solution isapplied to the nanoporous substrate to bind the perpendicularly alignedcarbon nanotubes to the nanoporous substrate.

In one embodiment, an aligned carbon nanotube array is provided. Thealigned array of carbon nanotubes includes purified carbon nanotubeswith approximately uniform diameters, a nanoporous substrate, and abinding agent. In this embodiment, the carbon nanotubes areelectrostatically-aligned perpendicular to a surface of the nanoporoussubstrate.

In another embodiment, a device for forming vertically aligned carbonnanotube arrays is provided. The device includes a first electrodeconnected to a high voltage source and a second electrode connected toground. The device also includes a substrate, onto which an applicationof a liquid suspension containing carbon nanotubes is applied. Thesubstrate is located between the first and second electrodes and is incommunication with the first electrode. The device also includes abinding solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a carbon nanotube aligning process inaccordance with an embodiment of the present invention.

FIG. 2 is a drawing of an exemplary electrostatic alignment device.

FIG. 3 is a drawing of an exemplary electrostatic alignment device.

FIG. 4 is a drawing depicting a vertically aligned carbon nanotube arrayproduced by an exemplary electrostatic alignment device.

FIG. 5 is a photograph depicting a cross-section of a vertically alignedsingle-walled nanotube deposit.

FIG. 6 is a photograph depicting a cross-section of a vertically alignedsingle-walled nanotube deposit.

DETAILED DESCRIPTION

The methods of the present invention provide processes to align carbonnanotubes (CNTs) from liquid dispersions. In one example, the methodvertically aligns carbon nanotubes post-growth. The carbon nanotubes maybe grown or produced by any method.

The disclosed methods effectively separate the carbon nanotube growthprocess from the alignment process. Post-growth, particular nanotubesmay be purified and selected for vertical alignment based on one or moreparameters. Further, the methods use an electrostatic field to aligncarbon nanotubes suspended in a solution, while the solution is removed.The CNTs can be single-walled nanotubes (SWNTs) or multi-wallednanotubes (MWNTs). Additionally, the CNTs may be any length; however,the length may be predetermined based upon the intended use of thealigned array. In one embodiment, the lengths range from approximately15 nm to 1 μm. However, preferably, the CNTs range in length fromapproximately 50 nm to approximately 1 μm.

The CNTs may have any diameter; however, a particular diameter may beselected based upon the intended use of the aligned array. In oneembodiment, the array contains CNTs ranging in diameter fromapproximately 0.5 nm to approximately 100 nm. In another embodiment, theCNTs are uniform in diameter. Preferably, the CNTs have an outerdiameter of at least 0.7 nm.

FIG. 1 is a flow diagram of a carbon nanotube aligning process 100. Atstep 102, the nanotubes are suspended in a liquid. By way of example andnot limitation, the CNTs may also be suspended in water, an alcohol,dimethylformamide (DMF), dimethyl sulfoxide (DMSO), aqueouspolyfunctional amine solutions (e.g. m-phenylenediamine), a latexpolymer, or a liquid containing one or more monomers. In a preferredembodiment, the CNTs are suspended in a solvent containing apolyfunctional amine.

The CNTs are suspended in the liquid at a concentration ranging fromapproximately 0.1 mg/ml to approximately 1.0 mg/ml. Preferably, thesuspension has a CNT concentration from approximately 0.1 m/ml toapproximately 0.3 mg/ml. In one embodiment, the CNTs in the suspensionhave the same length. In another embodiment, the CNTs in suspension maybe any length.

In the suspension, the CNTs adhere to one another due, in part, to vander Waals forces, residue from one or more catalysts, and otherbyproducts from the CNT production process. Thus, at step 104, thenanotube suspension is oxidized and/or dispersed to break up anynanotube bundles that may have formed. In one embodiment, the suspensionis dispersed using ultrasonic energy. The CNT suspension is subjected toultrasonication for up to 24 hours.

In another embodiment, an oxidant, such as ammonium persulfate, is addedto the suspension to aid in breaking up the nanotube bundles. Forexample, ammonium persulfate is added at a concentration ofapproximately 10 to 100 mg/ml of liquid. The CNT suspension includingthe added oxidant is then subjected to ultrasonication for up to 24hours.

At step 106, the oxidized suspension is washed and centrifuged tocollect the separated CNTs. In an embodiment, the oxidized suspension iswashed with deionized (DI) water a number of times and spun in acentrifuge.

At step 108, the supernatant liquid is removed, and the precipitatecontaining the CNTs is collected. By way of example and not limitation,the supernatant liquid may be removed by evaporation, decantation, orwithdrawn with a pipette. At step 110, one or more surfactants dilutedin water are added to the precipitate fraction in order to resuspend theCNTs. The surfactant or combinations of multiple surfactants tend tosuspend different CNTs based upon the diameter of the nanotube.Therefore, the surfactant(s) are specifically chosen to provide asuspension of CNTs having a preferred diameter. In various embodiments,the one or more surfactants may be diluted by other liquids. In otherembodiments, undiluted solvents may also be added to the precipitatefraction.

By way of example and not limitation, a mixture of water and anionicsurfactants, such as sodium dodecylsulfate (SDS) and sodium cholate(SC), is used to disperse CNTs of a particular diameter. In otherexamples, the surfactant may be sodium dodecylbenzenesulfonate, sodiumdeoxycholate, a cationic surfactant, a nonionic surfactant, orcombinations thereof. The CNT and surfactant mixture is centrifuged toseparate the non-dispersed CNTs from the dispersed CNTs having theparticular diameter, at step 112.

At step 114, the diameter-selected CNTs are collected and resuspended ina liquid that transitions to a gaseous phase at temperatures andpressures that do not adversely alter the stability of nanotubes. Forexample, the CNTs are suspended in a liquid that evaporates attemperatures and pressures lower than the melting point and/orsublimation point of the CNTs, such as water, ethanol, dimethylsulfoxide (DMSO), dimethyl formamide (DMF), or combinations thereof. Inone embodiment, the diameter-selected CNTs are suspended in an aqueouspolyfunctional amine solution (m-phenylenediamine). In a preferredembodiment, the diameter-selected CNTs are suspended in water.

The solution enriched with the diameter-selected CNTs is then depositedonto a nanoporous substrate mounted onto a flat plate electrode, at step116. Depositing the solution onto the nanoporous substrate may includecoating, spraying, or any other method of applying the solution to thenanoporous substrate. The flat plate electrode is connected to a voltagesource, such as a high voltage generator and positioned opposite asimilarly-shaped grounded electrode. At step 118, a non-reactive gas,such as nitrogen (N₂), is passed over the substrate. In one embodiment,the non-reactive gas is applied directly to the substrate using an airknife. In another embodiment, the non-reactive gas is used to purge anenvironment around the substrate and electrode. In this embodiment, thesubstrate and electrode may be placed within a closed environment orsuitable housing.

At step 120, a high voltage electrostatic field is generated between thetwo electrodes. The CNTs are aligned in the same direction as theapplied electrostatic field; therefore, the CNTs are alignedperpendicular to the nanoporous substrate.

The solvent is then removed at step 122. In one embodiment, the solventis removed by evaporation. For example, the nitrogen gas introduced atstep 118 aids the evaporation of the solvent suspending thediameter-selected CNTs. As the solvent evaporates, the solution on thesubstrate becomes more viscous thereby holding the CNTs perpendicular tothe substrate. In another embodiment, the solvent is drawn away from theCNTs and through the nanoporous substrate by capillary action. In yetanother embodiment, the solvent is removed by a vacuum applied to theopposite surface of the substrate. In a number of embodiments, thesolvent is removed as the electrostatic field is applied.

Next, a binder solution is applied to the substrate, at step 124. Thebinder solution initiates an interfacial polymerization reaction with apolyamine of the CNT solution. This reaction forms a polyamide polymermembrane in situ. The polymer membrane permanently locks the CNTs in theperpendicular orientation.

The aligned CNT array may be used in a number of applications. Forexample, the array may used as a filter where molecules having diameterssmaller than the diameter of the CNT pores will pass down the length ofthe CNTs, while molecules and ions with larger diameters will beexcluded from passing through the CNTs.

In one embodiment, the polymer membrane is thinner than the mean lengthof the CNTs. In another embodiment, the thickness of the polymermembrane is equal to the mean length of the CNTs. In yet anotherembodiment, where the CNTs are of uniform length, the thickness of thepolymer membrane is equal to the length of the CNTs.

The substrate and CNT array may be used in a number of applications. Byway of example and not limitation, the array may be incorporated intoelectronics, sensors, reverse osmosis filtration systems, gaspurification systems, and other filter systems.

FIG. 2 is a drawing of an exemplary electrostatic alignment device thatmay be used to align CNTs in liquid dispersions. The electrostaticalignment device 200 includes electrodes 202 and 204 separated by a gap206. One electrode 202 is connected to a high voltage source 208, whilethe other electrode 204 is grounded.

In one embodiment, the electrodes 202 and 204 are substantially flat.Furthermore, the electrodes 202 and 204 have rounded corners and bevelededges, to prevent arcing between the electrodes. The electrodes 202 and204 are not limited in size and may have any surface area suitable tosustain an electrostatic field that is substantially perpendicular tothe surface of the electrodes. In one embodiment, the electrodes arecircular and approximately one-inch in diameter. In another embodiment,the electrodes 202 and 204 are rectangular in shape and approximately14″×16″ in dimension.

The electrodes 202 and 204 may be composed of any conductive material.In one embodiment, the electrodes 202 and 204 are composed of aconductive metal, such as steel or aluminum. In another embodiment, theelectrodes 202 and 204 are composed of nonconductive material that hasbeen coated with a conductive material, such as indium tin oxide (ITO)coated glass. Preferably, the electrodes 202 and 204 are composed of anon-corrosive metal, such as stainless steel. In various embodiments,the electrodes 202 and 204 may be covered with polytetrafluoroethylene(PTFE) tape, such as Teflon tape. The tape provides additionalprotection to prevent arching between electrodes.

In one embodiment, the electrode 202 is a flat plate electrode that hasa surface area large enough to support a nanoporous substrate 210. Thesurface area of the electrode 202 is sufficiently larger than thesurface area of the substrate 210. Therefore, the substrate 210 can beplaced near the center the electrode 202 where the electrostatic fieldis more uniform, and not subjected to distortions in the electrostaticfield near the edge of the electrode 202.

The nanoporous substrate 210 is a membrane having the features of ananoscale filter. Preferably, the pores define a tortuous path throughthe membrane, thereby impeding the passage of carbon nanotubes throughthe membrane. The pore diameters range from approximately 0.5 nm to100nm. In one embodiment, the pore diameter of the substrate 210 is lessthan or equal to the diameter of CNTs being aligned.

In various embodiments of the electrostatic alignment device 200, anabsorbent blotter 212, is positioned between the electrode 202 and thesubstrate 210. The blotter 212, which may be composed of absorbentcellulose, protects the electrode 202 and aids in the removal of liquidfrom a CNT-containing solution 214 applied to the substrate 210.

The gap 206 between the electrodes ranges between 1 and 60 mm. In oneembodiment, the gap 206 is approximately 40 mm In another embodiment,the gap 206 is variable and based upon the voltage applied across theelectrodes 202 and 204. In this embodiment, the gap 206 is increased ata ratio of approximately 1 mm per 1000-5000 volts; therefore, a gap of20 mm is used, when 20,000 to 100,000 volts are applied across theelectrodes 202 and 204.

The high voltage source 208 may be any voltage source capable ofgenerating an electric potential. Preferably, the high voltage source208 is a generator capable of generating an electric potentialdifference between the electrodes of approximately 30 kV and 70 kV.

FIG. 3 is a drawing of an exemplary electrostatic alignment device thatmay be used to align CNTs in liquid dispersions. The electrostaticalignment device 300 includes housing 302 and a removable lid 304. Inone embodiment, the housing 302 and removable lid 304 are composed of anon-conductive or insulating material in order to prevent arching fromthe exterior of the device 300. For example, the housing may be composedof plastic, ceramic, or glass. The housing 302 and removable lid 304include one or more openings (not shown) through which electricalconnections can be made to electrodes 306 and 308 located within thehousing 302.

The electrodes 306 and 308 are similar to electrodes 202 and 204 (seeFIG. 2). The electrode 306 is connected to a high voltage source 310 andthe ground electrode 308 is connected to ground. The electrodes 306 and308 are separated from the housing 302 and the lid 304 by one or morestandoffs 312. The standoffs 312 are composed of an insulating material.In one embodiment, the standoffs 312 are affixed to the housing 302 andlid 304. In another embodiment, the standoffs are removable. The numberof standoffs 312 is variable, depending upon the size and orientation ofthe housing 302, the lid 304, or the electrodes 306 and 308.

With the lid 304 removed, a substrate 314 is placed on the chargedelectrode 306. In one embodiment, a blotter (not shown) is placedbetween the substrate 314 and the electrode 306. A solution 316containing suspended CNTs 318 is then applied to the substrate 314. As avoltage is applied to the electrode 306, an electrostatic field 320 isgenerated between the electrodes 306 and 308. The electrostatic field320 flows from the electrode 306 to the ground electrode 308.

In another embodiment, the interior of the housing 302 is purged with anon-reactive gas, such as nitrogen gas, from a gas source. By way ofexample and not limitation, a gas source may include a tank or cylinderof compressed gas. The non-reactive gas prevents arcing between theelectrodes 306 and 308. The non-reactive gas also expedites theevaporation of the solvent from the solution 316. In yet anotherembodiment, the solvent is evaporated, by applying the non-reactive gasdirectly to the surface of the substrate 314. For example, nitrogen gasmay be directly applied using an air knife 322.

In an embodiment, the nitrogen gas is applied continuously as thevoltage is applied to the electrode 306. In another embodiment, thenitrogen gas is applied to the substrate in intervals.

FIG. 4 is a drawing of an exemplary electrostatic alignment device afterthe high voltage source 310 is disconnected from the electrode 306. TheCNTs 318 have aligned themselves perpendicular to the surface of thesubstrate 314. A binder solution is applied to the substrate. Thebinding solution undergoes an interfacial polymerization reaction at theboundary between the binder solution and the remaining solvent. Thisreaction produces a polymer membrane 400 that holds the carbon nanotubein the perpendicular orientation.

FIGS. 5-6 are photographs of a cross-sectional view of an embodiment ofan aligned CNT array. The photographs depict a structure having a highdensity array of CNTs. The aligned CNT array produced by one or moreembodiments of the methods disclosed herein, include purified carbonnanotubes electrostatically aligned perpendicular to a surface of thenanoporous substrate. The purified carbon nanotubes are secured in placeby a binding agent. As shown, in various embodiments the CNTs aredensely packed. For example, the CNTs are aligned in the nanoporoussubstrate at a concentration of at least 1×10¹¹ cm⁻².

An Exemplary Method of Producing a Vertically Aligned Carbon NanotubeArray

In an exemplary method for producing a vertically aligned CNT array, ananoporous membrane, such as a Sepro PS35 polysulfone nanoporousultra-filtration membrane is used a substrate to support the array. TheSepro PS35 membrane, which is normally hydrophobic, is treated withPolyethylene Glycol (PEG) to make it hydrophilic. The membrane is themounted onto a cellulose blotter and placed onto a flat-plate electrodewithin a plastic housing.

Next, a suspension of CNTs in an aqueous polyfunctional amine solution(m-phenylenediamine) is deposited onto the surface of the nanoporousmembrane on the electrode. A plastic lid is placed over the housing.Mounted on the inside of the lid is a matching ground electrode. The lidis placed on top of the box, and a ground wire is attached to the groundelectrode. The ground electrode is mounted such that it is approximately40 mm from the flat-plate electrode supporting the nanoporous membrane.

Next, dry nitrogen from an air knife is blown over the surface of thenanoporous membrane that includes the deposited CNT solution. As thenitrogen gas is applied, approximately 40 kV is applied to theflat-plate electrode supporting the nanoporous membrane substrate. Thevoltage creates an electrostatic field that flows from the flat-plateelectrode to the grounding electrode. The flow of nitrogen gasevaporates the water from the CNT dispersion, while the electrostaticfield aligns the CNTs perpendicular to the nanoporous membrane.

As the solvent evaporates from the CNT suspension, the solute becomesmore viscous. The CNTs are then held in the vertical orientation, evenafter the electric field is turned off. Next, a solution of trimesoylchloride in heptane is applied to the surface. The trimesoyl chlorideand m-phenylenediamine react to form an insoluble polyamide membranecontaining the CNTs. Due to the limited solubility of the reactants inthe trimesoyl chloride solution and m-phenylenediamine, the thickness ofthe polyamide membrane is self-limited. Further, defects in thepolyamide membrane self-repair as the reaction continues. As thereaction progresses, the CNTs become permanently trapped in thepolyamide membrane.

While the invention has been explained in relation to exemplaryembodiments, it is to be understood that various modifications thereofwill become apparent to those skilled in the art upon reading thedescription. Therefore, it is to be understood that the inventiondisclosed herein is intended to cover such modifications as fall withinthe scope of the appended claims.

What is claimed is:
 1. A device for forming vertically aligned carbonnanotube arrays, the device comprising: a first electrode connected to ahigh voltage source; a second electrode connected to ground; a substrateonto which a liquid suspension containing a plurality of carbonnanotubes is deposited, the substrate is located between the first andsecond electrodes and is in communication with the first electrode; anda binding solution.
 2. The device of claim 1, where the binding solutionbinds the plurality of carbon nanotubes in an orientation perpendicularto a surface of the substrate.
 3. The device of claim 1, furthercomprising a non-reactive gas source.
 4. The device of claim 1, wherethe high voltage source generates an electrostatic field between thefirst electrode and the second electrode.
 5. The device of claim 4,where the electrostatic field aligns the plurality of carbon nanotubesin an orientation perpendicular to a surface of the substrate.